Block Confirmations and Transaction Security

On proof-of-work blockchains like Ethereum Classic, transaction security is measured in block confirmations. Each confirmation makes it exponentially more expensive to reverse a transaction.

## What Is a Confirmation?

When your transaction is included in block N, it has one confirmation. When block N+1 is mined on top of block N, your transaction has two confirmations. Each new block adds another confirmation.

## Why Confirmations Matter

In PoW, miners compete to extend the longest chain. Occasionally, two miners find valid blocks at nearly the same time, creating a temporary fork. The network resolves this by following the chain with more cumulative work — the "heaviest chain."

Transactions in the shorter fork are returned to the mempool and may be re-included in a future block. This is a **chain reorganization** (reorg).

## Confirmation Guidelines

The appropriate number of confirmations depends on the value at stake:

| Transaction Value | Recommended Confirmations | Wait Time (~13s/block) |
|------------------|--------------------------|----------------------|
| Small (< 100 ETC) | 6 | ~78 seconds |
| Medium (100–10K ETC) | 20 | ~4.3 minutes |
| Large (> 10K ETC) | 40 | ~8.7 minutes |
| Exchange deposits | 40–100 | 8.7–21.7 minutes |

## The Economics of Reorgs

To reverse a transaction with N confirmations, an attacker must:

1. Mine an alternative chain starting from the block before the transaction
2. This chain must be longer than the current chain (requiring > 50% of network hashrate sustained for N blocks)
3. The cost scales linearly with N: more confirmations = more mining power × more time needed

After Ethereum's Merge, ETC's hashrate increased dramatically, making 51% attacks substantially more expensive than pre-Merge levels.

## MESS and Additional Protections

In 2020, following 51% attacks, ETC implemented MESS (Modified Exponential Subjective Scoring, ECBP-1100) to penalize deep reorganizations. MESS was later deprecated after the post-Merge hashrate increase made attacks economically infeasible.

## Compared to PoS Finality

Proof-of-stake chains like Ethereum offer deterministic finality — after a certain point, transactions cannot be reversed at all. PoW chains offer probabilistic finality — transactions become increasingly secure but are never mathematically irreversible. The tradeoff is that PoW finality requires no trusted validator set.